This invention relates to a novel method and an apparatus for operating a circulating fluidized bed system.
Circulating fluidized bed (CFB) systems, such as CFB combustors include a combustion chamber having a fast fluidized bed of particles therein. A particle separator connected to a discharge opening in the upper part of the combustion chamber, for separating solid particles from the suspension of flue gases and entrained solid material being discharged from the combustion chamber. One or several return ducts are connected between the particle separator and the lower part of the combustion chamber, for recirculating separated solid particles from the particle separator into the combustion chamber. A gas outlet is arranged in the particle separator for discharging flue gases.
Cyclone separators are commonly used as particle separators. A dip leg type return pipe recirculates the separated particles from the cyclone to the lower part of the combustion chamber. A loop seal is arranged in the return pipe in order to prevent gases from flowing from the combustion chamber backward into the cyclone therethrough.
The circulating fluidized bed reactors are used in a variety of different combustion processes. Depending on the process, different bed materials are fluidized and circulated in the system. In combustion processes particulate fuel such as coal, coke, lignite, wood, waste or peat, as well as other particulate matter such as sand, ash, sulfur absorbent, catalyst or metal oxides can be the constituents of the fluidized bed. The velocity in the combustion chamber usually is in the range of 3.5 to 10 m/s, but can be substantially higher.
Typically heat is recovered from fluidized bed combustion processes by heat transfer surfaces in the combustion chamber and in the convection section disposed in the gas path after the particle separator. The peripheral walls of the combustion chambers are usually made as membrane walls in which vertical tubes are combined by fins to form evaporating surfaces.
Additional heat transfer surfaces such as superheaters may have to be disposed within the upper part of the combustion chamber for e.g. superheating the steam.
Corrosion and erosion may thereby constitute problems in the high temperature and high flow velocity surroundings in the combustion chamber. Heat transfer surfaces have to be made of heat resistant material often protected by some erosion resistant material or some special constructions have to be utilized. Such heat transfer surfaces are very heavy and expensive, heat resistant material being expensive. Corrosion affects heat transfer surfaces in the gas space of a combustion chamber especially at steam/water temperatures over 400.degree. to 500.degree. C., when burning fuels containing gaseous chlorine and alkali components.
It may also be difficult to achieve desired superheating of steam at low load conditions. The combustion chamber exit gas temperature decreases with decreasing load and the superheaters in the convection section do not have enough capacity to provide the desired results. Additional superheaters arranged within the combustion chamber increase costs and control problems in the boiler.
There has been a further need to find new ways to add heat transfer surfaces into the system without having to increase the size of the combustion chamber, especially in pressurized applications. In pressurized applications it is even less desirable to have to add the heat transfer surfaces into the combustion chamber and thereby increase the size of the combustor, as it would lead to an increased size of the pressure vessel and huge rise in costs.
It has been suggested to use external heat exchangers (EHE) for increasing the superheating capacity. In EHE superheaters are arranged in a separate fluidized bed reactor with hot circulating solid material, which is introduced into the EHE from the particle separator. The suggested external heat exchangers would be large and expensive, heavy if constructed by uncooled structures, as well as, difficult to control. A more simple and less expensive solution is needed.
It has also been suggested (see U.S. Pat. No. 4,716,856) to include heat transfer surfaces in the recycling system of a circulating fluidized bed reactor. The heat transfer surfaces would be disposed in a fluidized bed of solid circulating material collected in a heat exchanger chamber formed in the bottom part of the return duct. Thus the circulating solid material would provide the additional heat needed for e.g. superheating without a need to arrange separate external heat exchangers. A loop seal in the return duct provides the gas seal needed between the combustion chamber and the particle separator. The solid material would be reintroduced from the return duct into the combustion chamber by overflow.
Control of heat transfer in the recycling system has however not been satisfactorily solved yet. It has been suggested to use the fluidizing gas to control the heat transfer. The fluidizing gas is however also used for reintroducing solid material by overflowing from the recycling system into the combustion chamber. It would therefore not be possible to independently control heat transfer or solid material circulation by fluidizing gas at different loads.
Reintroducing the solid material into the combustion chamber by overflow as suggested would keep a constant bed level in the heat exchanger chamber and prevent variations in bed height, which is a clear limitation of the system. It is desirable in many applications to be able to control the height of the bed in order to be able to control the gas sealing effect of the bed and the heat transfer in the bed.
Further due to the overflow a less efficient mixing of solid material and hence less efficient heat transfer is achieved in the heat exchanger chamber. Solid material introduced onto the surface of the bed is only partly mixed into the bed. Material not mixed into the bed is immediately discharged through the overflow opening, without heat transfer to the bed or heat transfer surfaces.
Still further, due to the overflow, large and heavy particles tend to more easily fall into the bed and less efficiently circulate in the bed; that is they tend to accumulate on the bottom of the heat exchanger chamber without being reintroduced into the combustion chamber. The large particles may cause problems in heat transfer, fluidization, and solid flow in the return duct, as well as cause erosion.